A prior art multiple output DC-to-DC converter is shown in FIG. 1. A voltage source VIN is coupled to a first terminal of a transformer primary winding L1, designated with a "dot," and coupled to deliver power to an integrated circuit chip controller 1. A second terminal of the primary winding L1 is coupled to a drain of an NMOSFET Q1. A source of the transistor Q1 is coupled to a first terminal of a resistor R1 and is coupled to deliver a current sensing voltage signal SENSE to the controller 1. A second terminal of the resistor R1 is coupled to a ground node. A gate of the transistor Q1 is coupled to be controlled by the controller 1 by a control signal OUT. A first terminal of a transformer secondary winding L2, designated with a dot, is coupled to the ground node. A second terminal of the secondary winding L2 is coupled to a first terminal of a transformer secondary winding L4, designated with a dot, and is coupled to an anode of a diode D1.
A cathode of the diode D1 is coupled to a first terminal of a capacitor C2, to a first terminal of a resistor R4 and to an input terminal to a low drop out regulator LDO1. A second terminal of the resistor R4 is coupled to the ground node. A second terminal of the capacitor C2 is coupled to the ground node. A ground terminal of the low drop out regulator LDO1 is coupled to the ground node. An output terminal of the low drop out regulator LDO1 is coupled to a first terminal of a capacitor C5 and coupled to an output voltage node VOUT3 (typically 3.3 volts). A second terminal of the capacitor C5 is coupled to the ground node.
A second terminal of the secondary winding L4 is coupled to a first terminal of a transformer secondary winding L3, designated with a dot, and coupled to an anode of a diode D3. A cathode of the diode D3 is coupled to a first terminal of a capacitor C4, to a first terminal of a resistor R3, to a first terminal of a capacitor C1, to deliver a voltage feedback signal FB to the controller 1 and coupled to an output voltage node VOUT1 (typically 5.0 volts). A second terminal of the capacitor C4 is coupled to the ground node. A second terminal of the resistor R3 is coupled to the ground node. A second terminal of the capacitor C1 is coupled to a first terminal of a resistor R2. A second terminal of the resistor R2 is coupled to deliver a compensation signal COMP to the controller 1. A second terminal of the secondary winding L3 is coupled to an anode of a diode D2. A cathode of the diode D2 is coupled to a first terminal of a capacitor C3, to a first terminal of a resistor R5 and to an input terminal to a low drop out regulator, LDO2.
A second terminal of the capacitor C3 is coupled to the ground node. A second terminal of the resistor R5 is coupled to the ground node. A ground terminal of the low drop out regulator LDO2 is coupled to the ground node. An output terminal of the low drop out regulator LDO2 is coupled to a first terminal of a capacitor C6 and to an output voltage node VOUT2 (typically 12.0 volts). A second terminal of the capacitor C6 is coupled to the ground node. A ground terminal of the controller 1 is coupled to the ground node.
The controller 1 is coupled to deliver the control signal OUT which controls the state of Q1 by controlling the voltage level at the gate of Q1. When Q1 is on, a current flows from VIN through L1, through Q1 and through R1 to the ground node. The signal SENSE is an input to the controller 1 for sensing the current level through Q1 and L1 by monitoring the voltage across R1. A coupled inductor comprises a primary winding L1 and secondary windings L2, L3 and L4.
Unlike a conventional transformer, the coupled inductor conducts current in either the primary or secondary windings, but not both simultaneously. The primary winding L1 is inductively coupled to each of the secondary windings L2, L3 and L4, such that when L1 is turned off by Q1, the energy stored in the inductor core is transferred to L2, L3 and L4. The polarity of the windings as shown in FIG. 1 use the "dot" convention wherein a current entering the terminal of L1 that is designated with a dot will induce a current to flow into the terminals of the secondary windings designated with dots when Q1 is turned off.
The currents induced in L2, L3 and L4 charge the capacitors C2, C3 and C4 through the diodes D1, D2 and D3 to a voltage level which depends, in part, upon the ratios of the number of windings that comprise each of L1, L2, L3 and L4 and upon the current through L1. The voltage across C4 is fed back to the controller through the feedback signal FB so that the controller may control the gate voltage of Q1 for maintaining VOUT1 at a constant voltage level (typically 5 volts). A low drop out linear regulator LDO1 is coupled to the capacitor C2 for forming the output voltage at VOUT3 (typically 3.3 volts). A low drop out linear regulator LDO2 is coupled to the capacitor C3 for forming the output voltage at VOUT2 (typically 12 volts).
Each of the terminals VOUT1, VOUT2 and VOUT3 may be coupled to power a load. However, the low drop out linear regulators LDO1 and LDO2 are required because when no load is coupled to either VOUT2 or VOUT3, the voltage level across the capacitors C2 or C3 would tend to rise or "pump up" due to leakage inductance of the inductors L2 and L3. Eventually, the output voltage would rise to a level that could cause device damage. Resistors R4 and R5 provide a minimum load that limits the voltage to which C2 and C3 could be pumped up were a load not present on VOUT2 or VOUT3. R4 and R5 are often necessary to avoid damage to LDO1 and LDO2, but they severely limit the system efficiency at light loads.
The low drop out voltage regulators LDO1 and LDO2 have the disadvantages of increasing the complexity of the system and limiting the efficiency of the system. In addition, such a system has the disadvantages of requiring the use of compensation components R2, R3 and C1 for maintaining stability in the feedback loop, and being unable to operate in a low power mode due to the need for the low drop out regulators, further limiting overall efficiency. Examples of circuits bearing resemblance to the one described above can be found in Linear Technology's Application Note 30, in FIG. 85, on page 42.
Yet another scheme is disclosed in a paper by Bruce D. Moore of Maxim Integrated Products, entitled "System-Engineered Portable Power Supplies Marry Improved Efficiency and Lower Cost." In this scheme, a secondary winding is inductively coupled to a filter inductor of a buck regulator. This secondary winding functions as a transformer secondary winding to generate a loosely regulated voltage source. This voltage source is then regulated by dual linear regulators which suffer from many of the disadvantages outlined above.
In addition, other types of converters are known such as variable switching frequency converters which employ a constant off-time or hysteresis. Constant off-time converters terminate the charge cycle when a particular inductor current level is achieved and the off-time is the same for each cycle. Hysteretic converters use the inductor current to terminate both the charge and the discharge cycles.
Also, the prior art includes various other multiple output DC-to-DC converter topologies wherein additional output voltages are generated by connecting a tapped inductor across the regulated outputs during the discharge phase of the switching cycle.
In addition, an example of a converter having constant on-time is part number MAX756/MAX757 made by Maxim Integrated Products. However, this implementation does not feed back the inductor current signal during normal operation. Only the output voltage is fed back making this a "constant on-time voltage mode control" converter. This makes the converter operation less stable and less predictable than desired.
What is needed is a multiple output DC-to-DC converter which overcomes the above described disadvantages.